The setup: "Obviously I don't want to belittle the events of 9/11," says Sarah Silverman, halfway through this show, three years after those events occurred. You could write a PhD about the tension in the pause that follows. Silverman has already demonstrated, amply, that she is willing to belittle anything.

The unsayable always has that strange cliff-edge allure, and quite a few comedians forage their material in no-go areas. Yet very few perform it with the teasing intelligence of Silverman. Take an early and entirely typical gag from this show: "I was raped by a doctor," she confides, with all the seriousness those words deserve, "which is kind of bittersweet for a Jewish girl." It's not the Jewish stereotype that really powers the laugh, or the perfect inappropriateness of "bittersweet", it's the flip of mood from: "Here's something you can't joke about" to, "Oh yes you can." Dangerous – and liberating.

Funny, how? Of course, jokes like this will offend some people. In that, Silverman was precocious. (This 1992 set is outrageously good for a 21-year-old.) But in many ways her big break did not come until 2001, when a furore ensued from a joke she made about "chinks" on Late Night with Conan O'Brien. The episode established her as the official scrutineer of liberal America's hypocrisies (many of which overlap with those of liberal Britain). "I want to be the first comic ever to shit on Martin Luther King," she says in this performance from her Jesus is Magic tour, before doing so actually rather gently.

And then there's her heroically blunt songs, such as You're Gonna Die Soon, performed to a group of octogenarians. Or another, I Love You More, which is a list of racial stereotypes: "I love you more than black people don't tip", "I love you more than Puerto Ricans need baths" and so on. If you can make jokes about your own people, this number seems to ask, why shouldn't you do it about others?

Indeed what Silverman shows us, over and over, is that people are usually much less tolerant than the rules they live within. And that admitting this is funny. Exhibit A is always Silverman herself: a lifelong depressive, adolescent bedwetter and, you suspect, a complicated person to be around.

In the movie, this performance is set within the story of a wildly narcissistic standup star called Sarah Silverman, who winds up ravishing her own reflection. It's as if she hates herself for loving herself so much. And then loves herself for her bravery in admitting it. And so on. But perhaps it is only the shallow, intolerant "Silverman" persona who has these problems? You get lost in wondering these things with Jesus Is Magic, and it is impossible to look away.